This invention relates to a device for recovering chemicals from flue gases and green liquor, i.e. the water solution of the smelt obtained from burning the waste liquor of sodium based pulping processes.
The waste liquor from sodium based sulfite cooking contains remarkable amounts of sodium chemicals, the recovery of which is profitable.
For this purpose the waste liquor has been concentrated by evaporating and burning in a recovery boiler. From the burning there have been obtained flue gases containing sulphur dioxide and carbon dioxide, and a smelt which is dissolved into water to form so-called green liquor which contains sodium sulfide. Sulphur dioxide has been washed out from the flue gases in a washer with a sodium carbonate solution made of green liquor. The sodium sulfite obtained can be used as such in sodium sulfite pulping.
The sodium carbonate solution has been obtained from green liquor by precarbonizing the green liquor first with washed and cooled flue gases, whereby sodium sulfide reacts with the carbon dioxide of the flue gases and sodium bisulfide is obtained.
Thereafter the precarbonated sodium bisulfide solution has been allowed to react with a sodium bicarbonate solution produced in the process, whereby the following reactions occur: EQU NaHS+NaHCO.sub.3 .fwdarw.H.sub.2 S+Na.sub.2 CO.sub.3 (1) EQU 2NaHCO.sub.3 .fwdarw.CO.sub.2 +H.sub.2 O+Na.sub.2 CO.sub.3 (2)
The first reaction (1) is desired; the competing reaction (2) tends to decrease the sodium bicarbonate content. Reaction (1) is more rapid and it produces hydrogen sulfide which is stripped by steam and burnt in a sulphur burner into sulphur dioxide which can be led together with other flue gases into the washer to produce sodium sulfite.
The amount of sodium bicarbonate used in the stripping stage is 1.4 to 1.7 times the equivalent amount to sodium bisulfide, whereby about 90 to 95% of the sulphur contained by the sodium sulfide of green liquor has been converted into hydrogen sulfide. Thus a minor amount of sodium bisulfide remains in the solution.
Thereafter the stripped solution has been taken into a crystallizer in which sodium carbonate and sodium bicarbonate have been crystallized from the solution. These crystals have then been dissolved in water and part of the solution thus obtained has been led into the flue gas washer where the sulphur dioxide of the flue gases reacts with sodium carbonate to produce sodium sulfite. Part of the solution has been led into a carbonation reactor where the sodium carbonate of the solution has been allowed to react with a large enough portion of the washed flue gases to increase the sodium bicarbonate content of the solution so much that when this solution is taken into the stripping stage, it contains sodium bicarbonate 1.4 to 1.7 times the equivalent amount to sodium bisulfide.
The object of this invention is to accomplish a simpler device than before for recovering chemicals from flue gases and water solution of the smelt obtained from burning the waste liquor of sodium based pulping processes.